In principle, such devices consist of sensors which measure angles or revolutions by using the inductance principle. With existing devices the needed induction coils are installed in the stator and rotor or, as low cost and mass production resolvers in the stator only. In the last-mentioned known device, which is the subject of WO 92/16994, the stator is only provided with an induction and a measurement coil whereas the rotor is equipped on its circumference with a number of pole surfaces of differing polarity which will interact with said stator coils, whereby the pole surfaces are formed in such a way as to present in each angle position of the rotor a reluctance which is specific for this position.
In a further and preferred embodiment of the last-mentioned resolver, the pole surfaces of the rotor, seen over the whole circumference, are continuously changing such that the air gap between the pole surface and the corresponding stator part will continuously differ in each angle position (continuous change).
Although this known resolver has rather low production costs, the accuracy of the determined angle positions of the rotor or the revolutions of the shaft is, in many applications, not precise enough.